1. Field of the Invention
The present invention relates to an electric motor drive control system and control method thereof and, more specifically, to an electric motor drive control system formed with a converter allowing variable control of a DC voltage.
2. Description of the Background Art
Conventionally, as a type of an electric motor drive control system driving an AC electric motor, a structure has been used in which a DC voltage variably controlled by a converter is converted to an AC voltage for controlling driving of an AC electric motor using an inverter (see, for example, Japanese Patent Laying-Open Nos. 2003-33071 and 2003-116280, hereinafter referred to as Patent Documents 1 and 2, respectively).
By way of example, Patent Document 1 discloses a motor controller including a converter as a PAM (Pulse Amplitude Modulation) circuit and an inverter as a PWM (Pulse Width Modulation) circuit for converting an output voltage from the PAM circuit to an AC voltage. Particularly, in the motor controller disclosed in Patent Document 1, by balancing the durability of switching elements in the converter and the inverter, the life of the controller as a whole is enhanced.
In the drive device disclosed in Patent Document 2, a current flowing in a reactor in a converter is calculated by dividing an output battery power demanded of a battery, which is obtained by converting motive power required of a motor to an electric power, by a voltage between terminals of the battery. In accordance with the calculated current, carrier frequency of transistors forming the converter is set such that converter loss is minimized, whereby energy efficiency of the drive device can be improved.
Japanese Patent Laying-Open Nos. 2003-348892 and 2001-238490 (hereinafter referred to as Patent Documents 3 and 4, respectively) disclose motor controllers in which a plurality of motors are controlled efficiently by sharing an output of a converter having a DC voltage control function among a plurality of motor driving circuits (inverters having motor control function). In the structures disclosed in Patent Documents 3 and 4, the DC voltage value output by the converter is changed in accordance with conduction ratio of each inverter or a motor load.
In the structures in which the variably controlled converter output voltage is converted to an AC voltage by an inverter for driving an AC electric motor, loss in each component of the system varies dependent on the voltage level of the output voltage of the converter, that is, the voltage on the DC-linked side of the inverter and, therefore, efficiency of the system as a whole may also vary.
Patent Documents 1 to 4, however, neither disclose nor suggest determination of the DC voltage value variably controlled by the converter in consideration of the efficiency of the system as a whole, in the electric motor drive control system having such structures as described above.